1. Field of the Invention
The present invention relates to a memory device in which a memory cell is formed using a memory element which stores information according to a state of electric resistance.
2. Description of the Related Art
Since a semiconductor nonvolatile memory such as a flash memory is small-sized and recorded data is maintained even after a power supply is turned off, it is widely used as a medium for recording a moving picture and audio.
With respect to a nonvolatile memory, further greater recording capacity and recording density have been in demand.
A nonvolatile memory having a structure capable of multi-value recording, specifically which can store data of 2 bits or more in one memory cell, has been proposed as a structure to obtain the above.
At this time, in a case where the data of 2 bits can be recorded, for example, a memory element constituting a memory cell can maintain four kinds of state.
A flash memory or a resistance change type memory in which information is recorded by a change in the resistance value of a memory element has been known as a memory in which the art of multi-value recording is obtained.
In the resistance change type memory, the recording of information is performed by applying a voltage pulse to the memory element, for example (refer to non-patent reference 1).
Then, the resistance value of the memory element is made to vary by changing the number of application of this voltage pulse to obtain the multi-value recording.
[Non-patent reference 1] “Novel Colossal Magnetoresistive Thin Film Nonvolatile Resistance Random Access memory (RRAM)” written by W. W. Zhuang et al., Technical Digest “International Electron Devices Meeting”, year 2002, on page 193.
However, since a writing operation in the flash memory is performed by injecting in stages an electric charge into a terminal called a floating gate, considerably large amount of time is required compared with a case of performing 1-bit recording (single-value recording) when the flash memory is used to perform the multi-value recording.
Further, when the multi-value recording is also performed using the above described resistance change type memory, resistance change in stages to obtain the multi-value recording is controlled by the number of application of the voltage pulse to the terminal, and therefore it takes time as well and it is difficult to accelerate the operation speed.
When an information recording operation is performed with respect to a memory cell capable of performing the above described multi-value recording, it is necessary to input a plurality of bits and to perform the electric charge injection or to perform the number of pulse application corresponding to the plurality of input bits, and therefore it is desired to perform the recording operation (the electric charge injection or the number of pulse application) according to a relation between the plurality of input bits and the multi-value information to be recorded, using a circuit which is not complicated and is excellent in area efficiency.
Accordingly, it is desired to obtain the memory device in which the multi-value recording can be performed at a high speed and a drive circuit thereof has a simple circuit structure and is excellent in area efficiency.
In order to solve the above described problems, the present invention is to provide a memory device in which recording of multi-valued data can be performed at a high speed and the recording of multi-valued data is made possible using a drive circuit of comparatively simple configuration.